Linear Compressor

ABSTRACT

A linear compressor comprising a cylinder, a hydraulically borne piston that can be longitudinally oscillated inside the cylinder, and a drive unit driving the movement of the piston. The compressor is characterized in that at least one of the lateral faces of the piston and the cylinder that face each other is provided with an abrasion-resistant bearing coating.

The present invention relates to a linear compressor, especially for use in order to compress refrigerants in a refrigeration device.

A linear compressor is known from U.S. Pat. No. 6,506,032 B2. In this linear compressor a circumferential hollow space is formed in the wall of the cylinder and is supplied with hydraulic fluid, and communicates with the internal space of the cylinder in which the piston moves by a plurality of openings distributed over the lateral wall of the cylinder. Hydraulic fluid penetrating from these openings into the cylinder chamber forms a cushion on which the piston slides without contact with the lateral wall. Since the cushion prevents friction contact between piston and cylinder, such a linear compressor can work for a long time in continuous operation, without friction wear resulting in a noticeable diminution in efficiency.

However, when the compressor is used in practice in a refrigeration device, an unexpectedly fast diminution in efficiency is apparent.

A linear compressor is known from U.S. Pat. No. 6,641,377 B2 in which the piston is not hydraulically borne. In order to achieve a precise orientation of the piston in the cylinder, it is proposed to apply a coating of a solid lubricant such as PTFE or a DLC layer to the surface of the piston or the cylinder before inserting the piston into the cylinder chamber, so that piston and cylinder can be fitted together and a clearance between them required for continuous operation is obtained by eroding the coating in a start-up phase of the compressor.

The object of the present invention is to develop the linear compressor according to the preamble so that it has a constant efficiency even during long-term operation in a practical application such as a refrigeration device.

The object is achieved in that at least one of the lateral faces of the piston and of the cylinder that face each other is provided with an abrasion-resistant bearing coating.

It is apparent in fact that signs of friction wear unexpectedly occur on cylinder and piston during operation of a linear compressor of the type referred to in the preamble in a refrigeration device after lengthy operation despite the hydraulic bearing, said signs of friction wear resulting in an escape of fluid, increasing over time, from the cylinder chamber through the gap between piston and cylinder wall. If in fact the hydraulic fluid required for maintenance of the cushion is branched from a high-pressure connection of the compressor, the consequence is that at the start and end of each operating phase of the compressor, if the pressure at the high-pressure connection of the compressor is lower than during continuous operation, the cushion is not sufficiently effective to prevent friction contact between the lateral faces of piston and cylinder. In order to prevent wear in these operating phases, the abrasion-resistant bearing coating is necessary.

Whereas in the case of the compressor from U.S. Pat. No. 6,641,377 B2 the DLC or PTFE bearing coating is not said to be abrasion-resistant, so that it is eroded, if a clearance between piston and cylinder which is desired for operation of the compressor is produced, it is important for the present invention that the bearing coating is also present if the clearance between the lateral faces of piston and cylinder required for efficient operation of the hydraulic bearing is present. Surprisingly it was found that the bearing coating does not reduce efficiency and operation of the compressor at optimum efficiency is achieved over its service life thanks to the bearing coating.

Further features and advantages of the invention are given in the following description of an exemplary embodiment, and with reference to the enclosed figures. These show:

FIG. 1 a perspective view of a linear compressor, and

FIG. 2 a section through the cylinder of a linear compressor.

The linear compressor shown in FIG. 1 in perspective view has a rigid frame, somewhat U-shaped in planar view, which is composed of three parts, namely two flat wall pieces 1 and an arch 2. A first membrane spring 3 is stretched between end sides of the arch 2 facing each other and the two wall pieces 1; a second membrane spring 4 of the same construction as the membrane spring 3 is attached to the end sides of the wall pieces 1 facing away from the arch. The membrane springs 3, 4 punched from the spring plate each have two elongated edge strips 7 which cover the end sides of the wall pieces 1 or of the arch 2, and four spring arms 5 which extend in a zigzag fashion from the ends of the edge strips 7 to a center section 6, on which they meet. The center section 6 has in each case three drill holes, two external ones, at which a permanent magnetic oscillating body 8 is suspended with the aid of screws or rivets, and a center drill hole through which projects, in the case of the membrane spring 3, a rod section 10 which is attached to the oscillating body 8 by, for example, being screwed to it.

The rod section 10 is connected to a transmission rod 9 via a flexibly bendable tapered section 11. A second tapered section 12 connects the transmission rod 11 in one piece to a piston rod 14 which engages in a pump chamber supported by the arch 2, is routed through a drill hole in an end wall of the pump chamber, and in the pump chamber 15 is connected to a piston 16 (see FIG. 2) which can move therein.

Two electromagnets with an E-shaped yoke and a coil wound around the center arm of the E are in each case arranged between the oscillating body 8 and the wall pieces 1 with pole shoes facing the oscillating body and serve to drive an oscillating motion of the oscillating body 8.

Since the piston rod 14 rigidly connected to the piston 16 is routed in the end-side drill hole in the pump chamber 15, the piston 16 is protected against canting, even if it expands only slightly in the direction of the backward and forward movement. The piston 16 hence occupies little space in the pump chamber 14, so that a large effective volume is achieved with small external dimensions.

The pump chamber 15 is surrounded in annular fashion by a hollow space 17, which communicates with the pump chamber 15 through a plurality of openings 18 in the side wall of the latter and which is supplied via a through hole 19 with compressed gas tapped from a pressure connection 20 of the pump chamber. The compressed gas penetrating through the openings 18 into the pump chamber 15 forms a cushion on the side wall, on which the piston 16, whose diameter is slightly less than the free diameter of the pump chamber 15, slides essentially friction-free.

The shell of the piston 16 sliding along the wall of the pump chamber 15 is coated with a DLC (diamond-like carbon) layer 21. The DLC layer 21 can in particular be a tetrahedral carbon layer (ta-C) or an amorphous hydrogenous carbon layer (a-C:H). Such layers represent highly effective protection against friction wear in the case of extremely small thickness. This means that the thickness of the layer 21 can be significantly smaller than the radial clearance between piston and cylinder, so that the layer can be applied to a completed piston 16 whose dimensions are adapted to the pump chamber 15, without disadvantageous machining being required in order to adjust the dimensions. This permits an especially low-cost implementation of the invention, since the application of the DLC layer 21 to the piston 16 represents a single additional step in the manufacture of the compressor, which can be inserted easily into established production processes for such compressors, as it does not require any follow-up adjustments such as, for example, changes to dimensions in other manufacturing steps. 

1-5. (canceled)
 6. A linear compressor comprising: a cylinder including a cylinder lateral face; a hydraulically borne piston which moves in an oscillating fashion in a longitudinal direction of the cylinder and includes a piston lateral face; and a drive unit driving the movement of the piston, wherein at least one of the lateral faces of the piston and of the cylinder that face each other is provided with an abrasion-resistant bearing coating.
 7. The linear compressor as claimed in claim 6, wherein sufficient clearance exists between the at least one lateral face of the piston or of the cylinder bearing the bearing coating and the surface of the piston or of the cylinder that faces said lateral face, to permit a movement of the piston in the longitudinal direction without mutual touching of the lateral faces.
 8. The linear compressor as claimed in claim 6, wherein the lateral face provided with the bearing coating is a shell of the piston.
 9. The linear compressor as claimed in claim 6, wherein the bearing coating includes a DLC layer.
 10. The linear compressor as claimed in claim 6, wherein a branch for hydraulic fluid for the bearing of the piston is formed at a high-pressure connection of the compressor. 